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Time-Dependent Density Functional Theory Investigation of the UV–Vis Spectra of Organonitrogen Chromophores in Brown Carbon
journal contribution
posted on 2020-02-10, 15:11 authored by Jin Y. Chen, Emmy Rodriguez, Huanhuan Jiang, Kunpeng Chen, Alexander Frie, Haofei Zhang, Roya Bahreini, Ying-Hsuan LinThe
ability of brown carbon (BrC) in aerosols to absorb solar radiation
is an important but highly uncertain factor in climate forcing. The
uncertainties are partially due to incomplete characterization of
BrC chromophores and lack of authentic standards to confirm light
absorption. Organonitrogen species are crucial components in atmospheric
aerosols, but their light-absorbing properties remain to be fully
characterized. To facilitate the molecular characterization of BrC
chromophores, time-dependent density functional theory (TD-DFT) based
computational chemistry approaches were used in this study to predict
the light absorption spectra of 16 organonitrogen species, including
nitroaromatics, nitro-heterocyclic compounds, organonitrates, and
Maillard-type reaction products in BrC. Effects of basis sets, functionals,
solvation, and pH on light absorption properties of these compounds
were evaluated. Predicted absorption spectra were compared with experimental
measurements. Overall, the PBE0 and B3LYP functionals tend to outperform
PBE and CAM-B3LYP on the predicted absorption spectra of studied compounds.
Absorbance calculated in water and methanol (bulk solvents) varies
up to 2 nm (0.03 eV). Absorbance calculated in gas phase (vacuum state)
blue-shifts in comparison to solvation. Absorbance of weak acids (e.g.,
nitrophenols) is enhanced under basic conditions, and the absorption
spectra can be predicted by the fractions of conjugate acid–base
species. Results from this study demonstrate that a combined use of
TD-DFT predictions and experimental measurements of light absorption
can allow for a rapid and reliable determination of potential chromophores
in BrC when authentic standards are not available.